This invention relates to a method and apparatus for drying coating fluids and, more particularly, it concerns an improved drying arrangement including both corona and electrostatic drying units for reducing coating fluid drying time.
Coated products formed by applying a coating to a moving continuous web are well known in the photographic industry. Two of the major factors limiting maximum production rates and increasing manufacturing costs of such coated products are drying time and the application of multiple coatings to a single support web. A typical photographic coated product manufacturing facility includes large drying ovens and numerous coating runs. The large drying ovens provide for uniform drying and produce high quality photographic products. However, such ovens are costly to build and operate. The numerous coating runs provide for the sequential application of superposed coatings, especially solvent or oil based and aqueous coatings, since each coating must be dried before application of the next coating. Again, although the numerous coating runs produce quality multicoated products, such a multiplicity of equipment is costly to construct and energy and time consuming in operation.
Further, it is known to use corona generating or electrostatic field producing systems to assist in the application of a uniform coating on a charge-retaining base material of a photographic product. The uniformity of the coating enhances drying of the coating by allowing the drying oven to be set at an optimum temperature and, as such, reduces drying time. Examples of such systems are disclosed in commonly assigned U.S. Pat. Nos. 4,489,672 and 4,513,683 issued to Semyon Kisler and 4,457,256 issued to Semyon Kisler et al. Such coating systems are combined with the above-mentioned large drying ovens and coating runs to produce quality photographic coated products at optimum drying temperatures. Nevertheless, drying time still remains as one of the major factors limiting maximum production rates of such coated products.
One method for reducing the drying time of a coated photographic product described in U.S. Pat. No. 2,662,302 issued to Jack B. Cunningham et al involves the use of an ultra high frequency cavity resonator for dielectric heating and drying a dielectric photographic emulsion on a cellulose acetate, cellulose nitrate or paper base during photographic film manufacture. The photographic stock is continuously fed through the cavity resonator on a conveyor belt at a speed which allows the film to rapidly increase in temperature from about 32.degree. to 165.degree. F. as it passes through a concentrated high intensity field located in the center of the resonator and directed perpendicular to the emulsion surface. In order to subject the emulsion to a large increase in temperature without raising the emulsion temperature above a maximum of 165.degree. F., the cavity resonator is placed in a refrigerated room or housing maintained at 32.degree. F. so that the temperature of the film as it enters the resonator is about 32.degree. F. Thus, the dielectric heating system of Cunningham et al requires the use of not only a high frequency generator and refrigeration equipment, but also requires that the film be of a type not adversely affected by freezing temperatures.
U.S. Pat. No. 3,401,463 issued to Foster P. Doane, Jr. discloses a high speed paper making system which includes a high frequency dielectric heating unit for raising the temperature of water in a wet paper web prior to the web passing between a pair of press rolls. The dielectric heating unit is driven in a voltage range of 5 to 25 KV at a frequency between 2 to 90 megacycles to raise the water temperature to a maximum of 212.degree. F. Doane is concerned with raising the temperature of the water in the web without actually evaporating the water from the web and relies upon couch and press rolls to remove the dielectrically heated moisture from the paper web. Such a system is not conducive to the production of coated photographic products since physical contact with a wet coating and high temperatures typically have an adverse effect on the uniformity and characteristics of the coating.
Paper web drying systems including electrostatic drying arrangements are disclosed, for example, in U.S. Pat. Nos. 3,633,282 issued to Robert R. Candor et al and 4,359,826 issued to Robert R. Rounsley. The Candor et al patent discloses a paper web liquid removing apparatus including a plurality of opposing small and large electrodes used to produce nonuniform electrostatic fields oriented substantially perpendicular to the web. The electrostatic fields attract or drive moisture from the porous wet web in the direction of increasing intensity of the field. The Rounsley patent discloses a paper web drying system in which a porous wet web is subjected to evaporation energy in the form of heat produced by, for example, infrared energy, heated air, a heated drum, or radiant burners and enhancing the evaporation using electrostatic fields.
The use of corona devices for removing excess fluids is known. For example, U.S. Pat. No. 3,765,099 issued to Josef Kohlmannsperger discloses a method and apparatus for drying the wet surface of a photoelectric or dielectric copy carrier by using a gas-ion-air stream emanating from a set of transverse electrodes to push the liquid off the copy carrier during movement of the electrodes over the carrier. The air stream is developed by placing the wet copy carrier on a conductive support plate and providing a potential difference on the order of 4 to 8 KV between the electrodes and the conductive support plate. This rather large potential difference creates a relatively narrow and intense electric field between the electrodes and the underlying support plate and which passes through a relatively narrow transverse section of the wet copy carrier. The intensity of the electric field at the points of the electrodes causes ionization of the surrounding air and produces high speed gas ions directed toward the wet copy carrier. These moving gas ions form an air stream which drives the liquid off the wet copy carrier during movement of the electrodes over the length of the copy carrier.
U.S. Pat. No. 3,760,152 issued to Takashi Saito et al discloses a corona discharge device for removing excess dielectric liquid from a photosensitive or transfer sheet during an electrophotographic copy process. The corona discharge causes a narrow transverse strip of the dielectric liquid layer to be electrically charged progressively from the surface closest to the corona discharge device to the opposite surface of the liquid layer. The progressive charge creates a concave recess and self-squeezing effect in the liquid layer. As a result, excess liquid gathers in a bulged body on the sheet on the upstream side of the corona discharge. The corona discharge device includes a corona discharging electrode in a grounded shield with a narrow discharge opening such that only a very narrow transverse strip of the dielectric liquid surface is exposed to the corona discharge.
In light of the above, there is a need for an improved drying system for reducing coating-fluid drying time while at the same time eliminating the need for large energy and time consuming drying ovens and multiple coating runs, and which does not deleteriously affect the uniformity or quality of the coating.